Most etiologic studies on drug use, specially on injecting drug use, are based on treatment samples. This study aimed to clarify some potential differences between treated and untreated drug users that might be considered in studies that only have drug users recruited from treatment settings as participants. This study further shed light on different suspected determinants of entry into a detoxification treatment only and entry into a methadone maintenance treatment program. Our analysis were based on baseline and six-month follow-up interviews of initially 2,879 injecting drug users (IDUs), who had been recruited through extensive community outreach. We first examined characteristics associated with being in treatment at baseline and up to one year before baseline. A further investigation was focused on active injecting drug users, who had not been in treatment in that time period, and who returned for follow-up assessment within 9 months after baseline. We studied hypothesized determinants of those who entered a detoxification treatment, and those who entered a methadone maintenance program, comparing them to active IDUs who did not enter these treatments. Of 1,039 drug users, who reported injecting drugs at the time of the follow up interview, a total of 144 entered a detoxification program, a total of 64 entered a methadone maintenance treatment program. Multiple regression analysis indicated that enrollment in a detoxification program was associated with a recent episode of drug overdose, recent higher frequency of injecting drugs, and a history of arrest or treatment. Being married or living with a partner, female sex, long duration of drug use (>10 years), and a history of treatment had separate associations with enrollment into a methadone maintenance treatment program.